1. Field of the Invention
The present invention relates to a method of driving a liquid crystal matrix panel having a ferroelectric liquid crystal layer.
2. Description of the Prior Art
There has lately been reported the availability of a ferroelectric liquid crystal panel which has a high response speed and a memory function.
Hereinafter a conventional ferroelectric liquid crystal panel will be described. In a ferroelectric liquid crystal panel, slender molecules arrayed in layers are aligned at a fixed angle .theta. of inclination to a layer normal, and a dipole moment exists in the direction perpendicular to both the molecular major axis and the layer normal, so that the ferroelectric liquid crystal has a spontaneous polarization when formed into a thin film. ["Submicrosecond bistable electro-optic switching in liquid crystals" by N. A. Clark et al., Applied Physics Letters, Vol. 36 No. 11 (June 1, 1980), pp. 899-901]. When an electric field is applied to a ferroelectric liquid crystal cell shaped into a thin film, the molecules are placed in two states which are substantially horizontal to the substrate and have angles .+-..theta. of inclination to the layer normal depending on the direction of application of a voltage, so that light and dark tones can be realized by utilizing the birefringence or dichroism based on the electro-optical effect. An intermediate tone can be realized by a condition where the two states are intermingled and mottled in pixels. In the cell shaped to be sufficiently thin, the above two states are still maintained even after removal of the electric field due to the interaction of the molecules and the substrate surface. Such a cell is called a surface stabilized ferroelectric liquid crystal (SSFLC). ["FAST AND BISTABLE ELECTRO-OPTIC DISPLAYS USING FERROELECTRIC LIQUID CRYSTALS" by N. A. Clark et al., 1984 EURODISPLAY, pp. 73-76].
With regard to the matrix addressing method for the ferroelectric liquid crystal panel of the above-described structure, there is known a modification of the conventional amplitude selection scheme used for nematic crystal panels. ["An Application of Chiral Smectic-C Liquid Crystal to a Multiplexed Large-Area Display" by T. Harada et al., 1985 SID INTERNATIONAL SYMPOSIUM DIGEST of TECHNICAL PAPERS (May 1, 1985), pp. 131-134]. FIG. 1 shows a driving waveform in such a method. In this example, one scanning consists of two frames with pulses of mutually different polarities, and each frame is based on the 4:1 selection scheme. An AC voltage (.+-.V.sub.O) which can switch a pixel's state is applied to an ON pixel in an addressed period T.sub.O1 of the first frame, and a polarity-inverted AC voltage (.-+.V.sub.O) is applied to an OFF pixel in an addressed period T.sub.O2 of the second frame. In a non-addressed period T.sub.H of each frame, an AC bias voltage (.+-.V.sub.H) is applied to both pixels. It follows, therefore, that depending on the display pattern, the voltage applied in a non-addressed condition has double the pulse duration of each write pulse in an addressed condition. Since the molecules in the ferroelectric liquid crystal are movable more readily as the pulse duration becomes longer even when the applied voltage is the same, the pixel state is disturbed by the longer-duration pulses to consequently deteriorate the contrast. Although such deterioration is preventable by reducing the bias ratio, the on-off voltage ratio also becomes smaller so that proper display is rendered impossible unless the panel threshold characteristic is steep. Generally, when alignment is effected by rubbing, the voltage-to-transmittance characteristic becomes gentler in comparison with any cell aligned by shearing or the like through the process without substrate surface treatment, whereby the bias ratio cannot be reduced so much in the driving based on the conventional amplitude selection scheme used for nematic crystal panels. Practically, in an experiment conducted with regard to a panel where a mixture of ester ferroelectric liquid crystals is aligned by rubbing between substrates spaced apart from each other by a distance of 2.5 .mu.m, the contrast achieved was extremely low when such panel was driven by the conventional method of FIG. 1 at a multiplex ratio exceeding 100 under any conditions. The above problem is common to any driving method conforming to the conventional amplitude selection scheme used for nematic crystal panels. Furthermore, in the example of FIG. 1 where an on-pixel is written in the first frame and an off-pixel in the second frame respectively, an erroneous display is caused in the first frame in such a manner that new data overlaps the on-pixel of the preceding scanning, and when one scanning time is long or any motion picture is displayed, such overlap is visually seen so as to consequently bring about a deterioration of the display quality.
The surface stabilizing effect is diminished with an increase of the thickness of the cell. Relative to this problem, there is reported the effect of an AC field-stabilized state which is attained by applying a high-frequency AC electric field to a ferroelectric liquid crystal of negative dielectric anisotropy and thereby maintaining the molecules in parallel with the substrate by the dielectric anisotropy and the field effect. ["A Multiplexed Ferroelectric LCD Using ac Field-Stabilized State" by J. M. Geary, 1985 SID INTERNATIONAL SYMPOSIUM DIGEST of TECHNICAL PAPERS (May 1, 1985), pp. 128-130]. According to the above, first the entire matrix panel is once placed in its off-state and then an AC sinusoidal waveform is superposed on the driving waveform used in the conventional amplitude selection scheme. However, since the display is extinguished every time the picture plane is rewritten, the time required for appearance of the next display becomes different depending on the position in the panel if frequent sudden changes occur in the display, whereby a uniform display is rendered impossible. Furthermore, due to superposition of the AC waveform also on the write pulses in the addressed condition, the write pulse duration becomes short to bring about an undesired condition where the molecules are not readily movable.